To meet the demand for wireless data traffic having increased since deployment of 4th-generation (4G) communication systems, efforts have been made to develop an improved 5th-generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post long term evolution (LTE) System’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like. In the 5G system, hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, a machine-to-machine (M2M) communication, machine type communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet technology services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing information technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, MTC, and M2M communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud RAN as the above-described big data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology.
To achieve high data rates, deployment of the 5G communication system in a millimeter wave (mmWave) band (for example, a 60-GHz band) is under consideration. In order to mitigate propagation path loss and increase a propagation distance in the mmWave band, beamforming, massive MIMO, FD-MIMO, array antenna, analog beamforming, and large-scale antenna technology have been discussed for the 5G communication system.
Beyond voice-oriented service at their initial development stage, wireless communication systems are being developed to broadband wireless communication systems that provide high-speed, high-quality packet data services. For example, the broadband wireless communication systems may conform to communication standards such as 3rd generation project partnership (3GPP) high speed packet access (HSPA), evolved universal terrestrial radio access (E-UTRA), 3GPP2 high rate packet data (HRPD), ultra-mobile broadband (UMB), and institute of electrical and electronics engineers (IEEE) 802.16e.
A representative example of broadband wireless communication system, LTE adopts orthogonal frequency division multiplexing (OFDM) for downlink (DL) and single carrier frequency division multiple access (SC-FDMA) for uplink (UL), for broadband wireless communication. The multiple access schemes distinguish data or control information of users from one another by allocating and managing time-frequency resources for the users in such a manner that the time-frequency resources may not be overlapped for the respective users, that is, orthogonality may be maintained.
To increase transmission efficiency, the LTE system uses techniques including adaptive modulation and coding (AMC) and channel sensitive scheduling. A transmitter may control the amount of transmission data according to a channel state by using AMC. That is, if the channel state is poor, the transmitter may reduce the amount of transmission data, thereby adjusting a reception error probability to an intended level. On the contrary, if the channel state is good, the transmitter may increase the amount of transmission data, thereby effectively transmitting much information with an intended reception error probability. In channel sensitive scheduling, the transmitter services a user in a good channel state selectively from among a plurality of users. As a consequence, the wireless system capacity of the mobile communication system is increased, compared to a case in which the transmitter allocates a channel to a single user and services the user. The increase of the system capacity is called a multi-user diversity gain. In summary, AMC and channel sensitive scheduling are schemes of receiving a feedback partial channel state information (CSI) from a receiver and applying an appropriate modulation and coding scheme (MCS) at a time point determined to be most efficient.
If AMC is applied to an MIMO system, the number of spatial layers of a transmission signal or a rank, precoding, and the like may be considered. Specifically, when the MIMO system determines an optimum data rate using AMC, the number of layers for MIMO transmission as well as a coding rate and a modulation scheme may be considered.
To support AMC, a user equipment (UE) reports CSI to a base station (BS). The UE generates the CSI by measuring a reference signal (RS) received from the BS. The RS may include cell-specific RS (CRS) or channel status information-RS (CSI-RS). Time-frequency resources and a signal type to which the CRS and the CSI-RS are mapped are determined based on a pre-defined configuration.
The CSI includes at least one of a channel quality indicator (CQI), a precoding matrix indicator (PMI), or a rank indicator (RI). The CQI may represent a signal-to-interference and noise ratio (SINR) for a total system band (wideband) or a partial band (subband). In general, the CQI may be represented generally as a MCS for satisfying a predetermined data reception performance. The PMI may provide information about a precoding scheme required for data transmission through multiple antennas from a BS. The RI may provide information about a rank required for data transmission through multiple antennas from a BS. That is, the CSI is information that a UE provides to a BS to help the BS with scheduling decision. The BS may actually determine an MCS, a precoding scheme, and a rank to be applied to data transmission, based on the CSI.
Further, the LTE system adopts hybrid automatic repeat request (HARQ) in which if decoding of initial transmission data is failed, a physical layer retransmits the data. That is, the HARQ is a scheme in which if a receiver (for example, a UE) fails to accurately decode data, the receiver transmits a negative acknowledgment (NACK) indicating the decoding failure to a transmitter (for example, a BS) and thus the transmitter retransmits the data at the physical layer. As the receiver combines the retransmission data from the transmitter with the decoding-failed data, thereby increasing data reception performance. On the other hand, if the receiver accurately decodes the data, the receiver may transmit an acknowledgement (ACK) indicating the decoding success to the transmitter so that the transmitter may transmit new data.
Control information such as an HARQ ACK/NACK and CSI that the UE feeds back to the BS is called UL control information (UCI). In the LTE system, UCI is transmitted to the BS on a UL control channel dedicated to control information, physical uplink control channel (PUCCH), or multiplexed with data that the UE intends to transmit and transmitted to the BS on a physical channel for UL data transmission, physical uplink shared channel (PUSCH).
FIG. 1 illustrates a basic configuration of a time-frequency area being a wireless resource area, to which a DL data channel or control channel may be allocated in the LTE system according to the related art.
Referring to FIG. 1, the horizontal axis represents time, and the vertical axis represents frequency. In the time domain, a minimum transmission unit is an OFDM symbol. One slot 106 includes Nsymb OFDM symbols 102, and one subframe 105 includes two slots. One slot is 0.5 ms long, and one subframe is 1.0 ms long. A radio frame 114 is a time-domain unit including 10 subframes. A frequency-domain minimum transmission unit is a subcarrier, and a total system transmission bandwidth includes NBW subcarriers 104.
A basic time-frequency resource unit is a resource element (RE) 112, represented by an OFDM symbol index and a subcarrier index. A resource block (RB) or physical RB (PRB) 108 is defined by Nsymb contiguous OFDM symbols 102 in the time domain and NRB contiguous subcarriers 110 in the frequency domain. Therefore, one RB 108 includes Nsymb×NRB REs 112. In general, a minimum data transmission unit is an RB. In the LTE system, it is typical that Nsymb=7, and NRB=12, and NBW and NRB are proportional to a system transmission bandwidth. A data rate increases in proportion of the number of RBs scheduled for a UE. Six transmission bandwidths (refer to Table 1) are defined and managed in the LTE system. In a frequency division duplex (FDD) system in which DL and UL are distinguished from each other by frequency, a DL transmission bandwidth and a UL transmission bandwidth may be different. A channel bandwidth is a radio frequency (RF) bandwidth corresponding to a system transmission bandwidth. Table 1 illustrates a mapping relationship between system transmission bandwidths defined in the LTE system and channel bandwidths. For example, for an LTE system with a channel bandwidth of 10 MHz, the transmission bandwidth includes 50 RBs.
TABLE 1Channel BandwidthBWChannel [MHz]1.435101520Transmission Bandwidth Configuration615255075100NRB [Number of RBs]
Downlink control information (DCI) is transmitted in the first N OFDM symbols of a subframe. In general, N={1, 2, 3}. Therefore, N varies in each subframe according to the amount of control information to be transmitted in the current subframe. The control information may include a control channel transmission period indicator indicating the number of OFDM symbols over which the control information is transmitted, scheduling information for DL data or UL data, an HARQ ACK/NACK information, and so on.
In the LTE system, an evolved Node B (eNB) transmits to a UE scheduling information for DL data or UL data in DCI. Herein, UL refers to a radio link through which a UE transmits data or a control signal to an eNB, whereas DL refers to a radio link through which an eNB transmits data or a control signal to a UE. DCI is defined in various formats, and may indicate whether control information is scheduling information for UL data (a UL grant) or scheduling information for DL data (a DL grant), whether the DCI is compact DCI with a small size of control information, whether spatial multiplexing using multiple antennas is used, or whether the DCI is for power control, according to a predetermined DCI format. For example, Table 2 illustrates control information in DCI format 1, which is a DL grant.
TABLE 2Control informationDescriptionResource allocationResource allocation scheme. A basic schedulingtype flagunit is an RB defined by time and frequencyresources, and a resource block group (RBG)includes a plurality of RBs.0: allocates resources on an RBG basis byapplying a bitmap.1: allocates a specific RB within an RBGResource blockRB allocated for data transmissionassignmentRepresented resources are determined accordingto system bandwidth and the resource allocationschemeMCSModulation scheme used for data transmissionand the size of a transport block being intendedtransmission dataHARQ processNumber of HARQ processnumberNew data indicatorIndicates initial transmission or retransmissionRedundancy versionRedundancy version of HARQTransmit powerTPC command for a UL control channel, PUCCHcontrol (TPC)command for PUCCH
The DCI is transmitted on a physical downlink control channel (PDCCH) or an enhanced PDCCH (EPDCCH) after channel coding and modulation.
In general, the DCI is channel-encoded independently for each UE and transmitted on an independent PDCCH to the UE. The DCI is mapped to a control channel transmission period in the time domain. The position of a frequency area to which the DCI is mapped is determined according to the identification (ID) of the UE, and the frequency area is distributed across a total system transmission band.
DL data is transmitted on a physical channel for DL data transmission, physical downlink shared channel (PDSCH). The PDSCH is mapped after the control channel transmission period in the time domain. Scheduling information such as information about the position of a frequency area to which the PDSCH is mapped, and a modulation scheme for the PDSCH is included in DCI transmitted on the PDCCH.
The eNB indicates the modulation scheme applied to the PDSCH and the size of data to be transmitted (a transport block size (TB S)) to the UE by an MCS (5 bits) included in the DCI. The TBS is the size of a TB before channel coding is applied on the TB, for error correction.
The LTE system supports quadrature phase shift keying (QPSK), 16-ary QAM (16 QAM), 64 QAM, and 256 QAM as modulation schemes. These modulation schemes have modulation orders Qm of 2, 4, 6, and 8, respectively. That is, 2 bits per symbol may be transmitted in QPSK, 4 bits per symbol may be transmitted in 16 QAM, 6 bits per symbol may be transmitted in 64 QAM, and 8 bits per symbol may be transmitted in 256 QAM.
If the radio link quality between a transmitter and a receiver becomes poor to or below a predetermined level in a wireless communication system, data transmission and reception may not be performed normally. Therefore, a UE or an eNB determines whether a radio link failure (RLF) has occurred by monitoring a radio link quality (this operation is referred to as radio link monitoring (RLM)), and performs an operation corresponding to the determination.
FIG. 2 illustrates a procedure for determining whether an RLF has occurred by a UE in the LTE system according to the related art.
The UE accesses an eNB and transmits and receives data to and from the eNB during an active period 208. If the radio link quality between the UE and the eNB becomes poor continuously during a predetermined listening interval (listening period) 210, the UE recognizes that a problem has occurred to a radio link at time 201. If the radio link quality is not recovered during a predetermined time period T1 212, the UE may determine that an RLF has occurred at time 202. Upon occurrence of the RLF, the UE attempts to access a cell (i.e. a BS) having the best radio link quality from among neighbor cells during a predetermined time period T2 214. Once the UE accesses the cell having the best radio link quality, the UE continues data transmission and reception with the cell. On the contrary, if the UE fails to access the cell, the UE may end all transmission and reception operations and transition to an idle state 216.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.